A conventional transmission device and a conventional reception device doing a data communication in between can work with the same carrier frequency and with the same modulation and demodulation methods. Before starting a data communication, the transmission device and the reception device determine a carrier frequency and a modulation method and a demodulation method to be used between them.
A transmission device and a reception device, which carry out a data communication in bursts by the time division multiple access (TDMA) method, change a modulation method and a demodulation method or a rate of an error-correcting code one after another for data in a burst in response to a propagation environment. This is disclosed in Japanese Patent Unexamined Publication No. H07-250116 for expecting an improvement in transmission quality.
Between a base station and a terminal in a cellular system, the following technique has been adopted: When a terminal moves from a cell to another cell, the base station instructs the terminal to change a radio channel in a hand-over, so that a carrier frequency can be switched to another one even during a call.
Japanese Patent Unexamined Publication No. H08-130766 discloses a transmission device and a reception device both of which are equipped with plural communicating sections corresponding to respective communication methods such as TDMA, CDMA, and FDMA. Those communication methods are allotted to respective time slots in one data frame to carry out a communication.
As discussed above, a transmission device and a reception device employed in a general radio transmitting system use a communication method such as a carrier frequency, a modulation method and a demodulation method as a standard. The conventional transmission and reception devices disclosed in the foregoing Japanese Patent Unexamined Publications H07-250116 and H08-130766 use plural methods of modulation and demodulation; however, combinations of a rate of error correcting code and a modulation/demodulation method are predetermined by a communicating section prepared. When communication quality lowers, a combination is selected appropriately to the transmission from among the combinations prepared.
In the case of the conventional transmission and reception devices which switch a carrier frequency to anther one, the carrier frequency can be changed within a range of the frequency band allotted to the radio transmitting system used by those devices. A type of the modulation method and demodulation method and a band of the carrier frequency to be used by those conventional transmission and reception devices are thus confined to specific ones.
This situation allows a third device, having the same function as the transmission and reception devices, other than the devices supposed to receive data from the device which transmits the data, to intercept the radio wave running through the radio propagation channel. The radio wave intercepted undergoes a signal conditioning or an analysis, so that a wiretapping is achievable.